Steam-producing plants in which a combustion is effected in a circulating fluidized bed system and fluidized bed coolers are used for a recovery of the thermal energy contained in the combustion residue are known and have been described in European Patent No. 0 046 406 and in the European Patent Publication No. 0 033 713. Details of such plants are also known from U.S. Pat. Nos. 3,672,069; 4,111,158; and 4,165,717.
If the water vapor produced in such plants is utilized to produce power in a power plant, any changes influencing the steam production must be detected in time so that control actions can be taken quickly and the rate of steam production will be as constant as possible.